1. Field of the Invention
The present invention relates to an apparatus and a method for controlling a tracking servo in a near field optics, and more particular to an apparatus and a method for removing a dc offset of a one-beam push pull signal in a near field optics.
2. Description of the Related Art
Digital versatile discs (DVD), which are capable of recording and storing high definition video data and high sound quality audio data with a high capacity, are widely used, and it is expected that blu-ray discs (BD), which have a higher recording density as compared with the DVD, will be developed and commercialized.
The DVD has a storage capacity of about 4.7 GB, and the BD has a storage capacity of about 25 GB. Recently, next generation high definition discs such as a near field recording disc or a near field disc having a recording density much higher than that of the BD are being researched.
The near field disc may be called another name and has recording capacities of 140 GB to 160 GB. As shown in FIGS. 1 and 2, since a numerical aperture (NA) of an objective lens provided in an optical pickup for the near field disk must have a higher value than that for the BD in order to record data in high definition, a solid immersion lens 11 in a hemisphere shape is formed in front of an objective lens 10, thereby increasing the numerical aperture.
In addition, the tracking servo scheme for an optical disc includes a one-beam push pull scheme as shown in FIG. 3. A laser beam reflected from the optical disc passes through an objective lens, is incident on a photo detector 20 which is divided into an A area and a B area, and is converted into an A signal and a B signal in the A area and the B area of the photo detector, respectively, in order to detect a push pull signal. Then, a difference between the two signals is used as a tracking error signal (TE=(A−B)).
Then, a tracking servo operation, which moves the objective lens right and left in a direction parallel to a surface of the optical disc, is performed, so that the tracking error signal becomes a minimum value (e.g., TE=0). According to the one-beam push pull scheme, when the objective lens is moved as described above, a laser beam incident on the photo detector is moved right and left, so that an optical offset is caused between the A area and the B area.
Since the optical offset caused as described above is not divided from a tracking error in a state where a tracking servo is turned on, the optical offset is a main factor causing a tracking servo error such as a “de-track” phenomenon in which a laser beam deviates from a center of a track in an optical disc having eccentricity.
In order to solve the problem, a differential push pull scheme using one main beam, two sub beams, and three photo detectors 30, 31, and 32 may be employed as shown in FIG. 4. According to the differential push pull scheme, while using the A signal and the B signal derived from the main beam as a push pull signal (A−B), an offset value included in the push pull signal, that is, the DC-offset component is removed by combining an E signal and a G signal and an F signal and an H signal derived from the two sub beams with each other in the form of “k{(E+F)−(G+H)}”, and subtracting the “k{((E+F)−(G+H)}” from the push pull signal (A−B).
In addition, a tracking servo operation allowing the objective lens to move right and left in a direction parallel to a surface of the optical disc is performed, so that the tracking error signal (TE={(A−B)}−k{(E+F)−(G+F)}, in which the dc offset component is removed, becomes a minimum value (e.g., TE=0).
However, as described above, one main beam, two sub beams, and three photo detectors must be used through the differential push pull scheme. To this end, the optical disc must be spaced from the objective lens with a predetermined distance. Accordingly, it is difficult to apply the differential push pull scheme to near field recording in which data must be recorded and reproduced in a state where the objective lens is much close to the optical disc.
In addition, as described above, since the dc offset component cannot be removed when the one-beam push pull scheme is applied, an error occurs in the tracking servo operation.